In my Markdownify Instagram post, I used regular expressions to create a Python dictionary out of information in a text file. While we're on the topic, this is a nice collection of regular expressions you can use to filter your tweets. Helpful for removing some of the garbage people post on Twitter. The hashtag abuse ones are particularly useful. And if you like those, there's even more here.

I believe Tweetbot is the only Twitter client that has regex support. You can add a new mute filter by clicking on the mute button in the tab bar, then clicking "Edit" → "+" → "Mute Keyword", and sliding the Regular Expression slider to "ON" after you enter an expression in the field.